Remove UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true setting.

It needs to be controlled by build instead:

- Prebuilt SDKs should be used for unbundled Mainline module builds, and
  is necessary on thin module branches to avoid dependencies on
  frameworks/base (e.g. framework-doc-stubs used by lint-check).

- Source SDKs are needed on master which can otherwise get problems
  with out of date prebuilts (see e.g. b/140162931).

Test: env TARGET_BUILD_APPS=com.android.art \
        packages/modules/common/build/build_unbundled_mainline_module.sh \
        --product module_arm --dist_dir out/dist
  using the thin master-art manifest
Bug: 202192894
Change-Id: Ia73ccc6e694caec64c7dd0d86246a258e2bb9239
diff --git a/build/build_unbundled_mainline_module.sh b/build/build_unbundled_mainline_module.sh
index 3c7d78b..6c77592 100755
--- a/build/build_unbundled_mainline_module.sh
+++ b/build/build_unbundled_mainline_module.sh
@@ -118,8 +118,10 @@
 
   # This script cannot handle compressed apexes
   declare -grx OVERRIDE_PRODUCT_COMPRESSED_APEX=false
-  # Unset to build using PreBuilt SDK.
-  declare -grx UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true
+
+  # UNBUNDLED_BUILD_SDKS_FROM_SOURCE defaults to false, which is necessary to
+  # use prebuilt SDKs on thin branches that may not have the sources (e.g.
+  # frameworks/base).
 }
 
 function main() {